1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related mobile device, and more particularly to, a method of handling cell selection in the wireless communication system and related mobile device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
When a UE is switched on, a public land mobile network (PLMN) is selected by NAS (non access stratum, NAS). The PLMN may rely on several radio access technologies (RATs), e.g. UTRA and GSM. The NAS can control the RATs in which the cell selection should be performed, for instance by indicating RATs associated with the selected PLMN. The NAS provides a list of equivalent PLMNs. If any of PLMNs in the list is available, the access stratum (AS) use it for cell selection and cell reselection.
With the cell selection, the UE searches for a suitable cell of the selected PLMN and chooses the cell to provide available services, and further the UE shall tune to its control channel (i.e. establish BCCH, and PCCH). This choosing is known as “camping on the cell”. The UE then registers its presence, by means of a NAS registration procedure, in the registration area of the chosen cell, if necessary. When camped on the cell, the UE regularly searches for a better cell according to the cell re-selection criteria. If a better cell is found, that cell is selected.
When the UE has completed the cell selection/reselection process and has chosen a cell, the UE monitors system information. According to the system information, such as cell status or cell reservations, the UE can know if the cell is barred, or reserved.
Cell status and cell reservation information are indicated in the SystemInformationBlockType1 by means of three Information Elements, described as follows: (1) cellBarred (type: “barred” or “not barred”). (2) cellReservedForOperatorUse (type: “reserved” or “not reserved”). (3) cellReservationExtension (type: “reserved” or “not reserved”).
When the cell status of a cell is indicated as “not barred”, “not reserved” for operator use and “not reserved” for future extension (Cell Reservation Extension), all UEs shall treat this cell as candidate during the cell selection and cell re-selection procedures. When the cell status is indicated as “not barred”, “not reserved” for operator use and “reserved” for future extension (Cell Reservation Extension), the UEs shall behave as if the cell status “barred” is indicated. When the cell status “barred” is indicated, the UE is not permitted to select/re-select this cell, not even for emergency calls, and ignores the “Cell Reserved for future extension (Cell Reservation Extension) use” IE. In this situation, the UE shall select another cell.
However, those behaviors are only assigned to the UE which is operated in the RRC_idle mode. When the UE in the RRC_Connected mode reads SystemInformationBlockType1, the current specifications does not specify how the UE shall behave in the RRC_Connected mode in a cell with a barred or reserved status. As a result, problems may occur due to ambiguity.
In the first scenario, an E-UTRAN has a cell with the barred status to avoid the UEs in the RRC_Idle mode to camp on. In the meantime, the E-UTRAN intends to use the cell for handover in consideration of resource management. Namely, the UEs in the RRC_Connected mode can be moved to the cell by E-UTRAN indications. After the UEs in the RRC_Connected mode reads SystemInformationBlockType1 of the cell, the UEs all determine that the cell is barred. As a result, the handovers fail due to the barred status, thereby resulting in failure of the resource management plan for the cell.
In the second scenario, an E-UTRAN intends to use the cell for RRC connection re-establishment and designate the cell as a re-establishment cell. Therefore, the cell was set to the barred status to avoid the UE in the RRC_Idle mode to camp on. A UE in RRC connected mode in another cell has radio link failure and thereby initiates a RRC connection re-establishment procedure requesting cell re-selection. The UE in RRC connected mode only finds the re-establishment cell during the cell re-selection and thus selects the re-establishment cell. However, the RRC connection reestablishment fails since the UE determine that the cell is barred through the SystemInformationBlockType1. As a result, the resource management plan for the re-establishment cell fails.
In the third scenario, an E-UTRAN changes a cell status from the “not barred” status to the “barred” status for operation and maintenance purpose. In that cell, the UEs in the RRC_Idle mode select other cells to camp on. On the other hand, some UEs in the RR_Connected mode are determined to continue staying in the call for data transmission and reception due to implementation criteria of UE manufacturers. However, the E-UTRAN has already stopped data transmission and reception with the UEs in the RRC_Connected mode since the cell is barred for the maintenance purpose. This, thus, wastes power of the UEs in the RRC_Connected mode. In the other case, some of the UEs in the RRC_Connected mode are determined to switch to the RRC_Idle mode due to different implementation criteria of UE manufacturers. Therefore, the connection drops, causing transmission and reception problems.
To put it simply, when the cell is barred by the E-UTRAN indication, the current specification only specifies behaviors for those UEs in the RRC_Idle mode. Regarding to the UEs in the RRC_Connected mode, there has not been any regulation specified for the UE in the RRC_Connected mode to deal with the barred cell so far. Thus, without specification for the UE in the RRC_Connected mode, the problems, such as handover failure, radio link failure, extra power consumption and transmission suspension, occur.